Planet Formation with Migration

In the core-accretion model, gas-giant planets form solid cores that then accrete gaseous envelopes. Tidal interactions with disk gas cause a core to undergo inward type I migration in 10⁴-10⁵ yr. Cores must form faster than this to survive. Giant planets clear a gap in the disk and undergo inward type II migration in <10⁶ yr if observed disk accretion rates apply to the disk as a whole. Type II migration times exceed typical disk lifetimes if viscous accretion occurs mainly in the surface layers of disks. Low turbulent viscosities near the midplane may allow planetesimals to form by coagulation of dust grains. The radius r of such planetesimals is unknown. If r < 0.5 km, the core formation time is shorter than the type I migration timescale, and cores will survive. Migration is substantial in most cases, leading to a wide range of planetary orbits, consistent with the observed variety of extrasolar systems. When r ~ 100 m and the midplane α ~ 3 × 10^-5, giant planets similar to those in the solar system can form.